The delivery of radio frequency (RF) energy to target regions within solid tissue is known for a variety of purposes of particular interest to the present invention. In one particular application, RF energy may be delivered to diseased regions (e.g., tumors) for the purpose of ablating predictable volumes of tissue with minimal patient trauma. RF ablation of tumors is currently performed using one of two core technologies.
The first technology uses a single needle electrode, which when attached to a RF generator, emits RF energy from the exposed, uninsulated portion of the electrode. This energy translates into ion agitation, which is converted into heat and induces cellular death via coagulation necrosis. In theory, RF ablation can be used to sculpt precisely the volume of necrosis to match the extent of the tumor. By varying the power output and the type of electrical waveform, it is possible to control the extent of heating, and thus, the resulting ablation. The second technology utilizes multiple needle electrodes, which have been designed for the treatment and necrosis of tumors in the liver and other solid tissues. In general, a multiple electrode array creates a larger lesion than that created by a single needle electrode.
The size of tissue coagulation created from a single electrode, and to a lesser extent a multiple electrode array, has been limited by heat dispersion. As a result, multiple probe insertions must typically be performed in order to ablate the entire tumor. This process considerably increases treatment duration and patient discomfort and, due in large part to the limited echogenicity of the ablation probe when viewed under ultrasonography, requires significant skill for meticulous precision of probe placement. In response to this, the marketplace has attempted to create larger lesions with a single probe insertion. Increasing generator output, however, has been generally unsuccessful for increasing lesion diameter, because an increased wattage is associated with a local increase of temperature to more than 100° C., which induces tissue vaporization and charring. This then increases local tissue impedance, limiting RF deposition, and therefore heat diffusion and associated coagulation necrosis.
It has been shown that the introduction of saline into targeted tissue increases the tissue conductivity, thereby creating a larger lesion size. Currently, this is accomplished by treating the tissue with a separate syringe. See, e.g., Ahmed, et al., Improved Coagulation with Saline Solution Pretreatment during Radiofrequency Tumor Ablation in a Canine Model, J Vasc Interv Radio 2002, July 2002, pp. 717-724; Boehm, et al., Radio-frequency Tumor Ablation: Internally Cooled Electrode Versus Saline-enhanced Technique in an Aggressive Rabbit Tumor Model, Radiology, March 2002, pp. 805-813; and Goldberg et al., Saline-Enhanced Radio-Frequency Tissue Ablation in the Treatment of Liver Metastases, Radiology, January 1997, pp. 205-210. Treating the tissue with a separate syringe, however, is not the most efficient and least invasive manner to deliver saline to the target tissue, since it requires an additional needle insertion and does not anticipate the tissue locations where the ablations will ultimately be performed.
It has also been shown that, during an ablation procedure, a needle electrode can be used to perfuse saline (whether actively cooled or not) in order to reduce the local temperature of the tissue, thereby minimizing tissue vaporization and charring. A needle, however, typically cannot deliver the amount of saline necessary to significantly increase the conductivity of the tissue, either due to an insufficient number or size of perfusion openings within the needle electrode and/or the occurrence of clogged openings resulting from the entrapment of tissue during introduction of the probe.
Thus, there is a need for an improved ablation probe that can maximize the delivery of fluid to tissue in order to provide a more efficient, effective, and dynamic ablation treatment of tissue.